Link bundling is a technology that allows a device to group or bundle together multiple, dislike or like interfaces (referred to as component interfaces or links) that connect one router/switch to another under the umbrella of a single, virtual link bundle interface. This interface is then viewed by the device as a single pipe to that adjacent neighbor, consisting of the combined bandwidth and other attributes of the component links. For example, a single set of transmit and receive counters reflect the aggregate behavior of the individual component links, and provide a single point of management for the operator. This interface may also be introduced as a single interface to that neighbor to the routing protocols in place of the operator having to worry about attaching each individual component interface to the routing protocol(s). The routing protocols will then instruct the forwarding plane to forward all traffic destined for a particular next-hop to use the single virtual link bundling interface. However, under the covers, traffic will be load-shared across all component links in order to best share the combined bandwidth of all adjacent links. The algorithm used to share traffic across component links is identical to the one used for spreading the load across adjacent Interior Gateway Protocol (IGP) equal cost links, often referred to as Equal Cost Multi-Path (ECMP). The most common algorithms are: 1) per-packet, which uses some form of round-robin scheduling to push each packet over a different component link or 2) per-destination, which sends all packets destined to a particular IP destination over the same component link. There are advantages and disadvantages to using either algorithm, but the most commonly used algorithm is the per-destination algorithm.
Link bundles provide an optimization on link management for operators that may simplify their configuration and management of the Interior Gateway Protocol (IGP). One advantage is that the links need not be of the same access speed, media type, or otherwise related, just that they attach to the same routed neighbor. In most cases, this means physically connected routing neighbors. This allows the operator to upgrade or move adjacent links without having to worry much about capacity planning. Link bundling may also provide a non-stop forwarding capability whereby component links are removed or added to a virtual link bundle interface without disrupting the routing protocol, which in normal cases might occur when links are disabled. Link bundles also provide an optimization that simplifies (shortens) the number of Link State Advertisements (LSAs) that need to be sent around during routing updates, making routing updates faster.